1. Field of the Invention
The present invention relates to an automatic focusing system and, more particularly, to an automatic focusing system using automatic focusing based on an optical path length difference method of automatically performing focusing on the basis of contrasts in images of a subject imaged on a plurality of image pickup surfaces with optical path length differences.
2. Related Art
An automatic focusing (AF) method called an optical path length difference method has been proposed as an AF method suitable for video cameras such as television cameras for broadcasting (see, for example, Japanese Patent Application Laid Open No. 2002-365517). In an AF system using AF based on the optical path length difference method, a light splitting optical system such as a half mirror is placed in a picture taking lens to form an AF optical path branching from the main optical path, and part of subject light entering the picture taking lens travels through the AF optical path. An image pickup device used in the main section of the camera for obtaining a recording or reproduction picture signal (referred to as “picture image pickup device” in this specification) is placed in the main optical path. A recording or reproduction picture signal is obtained by the picture image pickup device.
On the other hand, a plurality of image pickup surfaces of an image pickup device for AF (referred to “AF image pickup device” in this specification) are placed in the AF optical path with optical path length differences defined therebetween. A plurality of image pickup surfaces may be formed by a plurality of AF image pickup devices or by one AF image pickup device. Subject light in the branching AF optical path is divided by the light splitting optical system disposed in the AF optical path to reach the image pickup surfaces of the AF image pickup device. A subject image is thereby formed on each image pickup surface and a picture signal is obtained from the AF image pickup device with respect to each image pickup surface. Contrasts in the subject images imaged on the image pickup surfaces are obtained as focus evaluation values on the basis of the picture signals corresponding to the image pickup surfaces. The focus evaluation signals are compared with each other to obtain the state of focusing of the picture taking lens with respect to the image pickup surface of the picture image pickup device (correct focus, front focus or rear focus), and focusing of the picture taking lens is controlled so that the picture taking lens is correctly focused.